1. Field of the Invention
The present invention relates to a method for reversible imaging of a printing form by image controlled heating of a surface by means of thermal transfer. More particularly, the present invention provides a method for reversibly imaging a printing form by means of heat treating a thermal transfer material to ensure uniform printing quality without having to adapt the thickness of the layer of the thermal transfer material according to the desired impression and to simplify erasure of the image from the printing form.
2. Description of the Related Art
There are known printing methods in which a printing form is provided with thermal transfer material according to an image by a thermal transfer film. The printing form is preferably on a printing form cylinder. Transfer of the transfer material is preferably laser induced. Other energy sources, such as heating elements, heating lines, heating matrices or the like may also be used. The printing form is then inked with printing inks, in particular for an offset method. The printing ink of the ink-carrying regions is transferred, if appropriate, via a rubber roller onto the substrate to be printed.
In order to change the printing subjects quickly it is desirable to perform the operation within the printing machine by computer control without moveable parts being changed. This is particularly true for small impressions.
German reference DE 38 09 915 A1 discloses a printing method where image information in the form of ink-absorbing surface elements transfers image information onto the lipophobic surface of the printing form or the printing form cylinder via an image information transfer unit within the printing machine. A thermal transfer film is provided with a thermosensitive or electrothermosensitive coating which has oleophilic or ink-absorbing properties. The image information transfer unit includes a printing head such as a line of heating elements, an electrode, an energy beam or any other heat-generating unit, in particular a laser printing head. In order to transfer an image information item, the printing head is controlled via corresponding image signals, in such a way that, for each image dot, it introduces heat and pressure to the thermal transfer film and consequently causes a punctiform transfer of the coating of the film onto the surface of the printing form cylinder. At the same time the printing form cylinder rotates, and the printing head is correspondingly traversed, so that the printing form can be imaged, for example spirally, on the printing form cylinder by the thermal transfer film.
For a repeated image-conforming coating of a printing form of this type, subassemblies are arranged within the printing machine. The subassemblies consist of a means for supplying a thermal transfer film to the printing form cylinder, a laser printing head capable of being coordinated with the rotational movement of the printing form cylinder, an electronically controlled image-spot transfer unit for activating the laser printing head and an element which removes the image-conforming coating from the printing form again. The subject presented in European reference EP 0 698 488 B1 fulfills this requirement. As shown in German reference 196 24 441 C1, the element for removing the image-conforming coating or the thermal transfer material from the surface of the printing form may be a high-pressure cleaner.
The strip-like thermal transfer film disclosed in European reference EP 0 698 488 B1 is distinguished by a comparatively thin coating of thermal transfer material. The imaging layer on the printing form cylinder is therefore thin, so that the image-conforming coating can also be readily removed, i.e. the printing form cylinder can be erased again more easily or more quickly due to the reduction in thickness of thermal transfer material.
However, it is also known that the impression constancy of a printing form imaged by thermal transfer, or the uniform printing quality over the entire number of copies of a specific printing product to be printed, depends directly on the layer thickness of the thermal transfer material. When the thickness of the thermal transfer layer is increased to improve impression constancy, then removal of the image-conforming coating from the printing form during erasure becomes difficult, resulting in ink streaks or ghost images, i.e. xe2x80x9cscummingxe2x80x9d occurs on the printing form or in the printing image.
The goal is to ensure that the printing form coated in conformity to the image affords an adequate service life for as large an impression as possible and to obtain uniform printing quality. In addition, it is important to assure that the thermal transfer material can be removed from the printing form after the printing operation, in a simple environmentally friendly manner, so that a new imaging and printing operation can commence immediately.
For this reason, attempts have already been made to use a thin-layer thermal transfer film in the printing method described above but then to perform infrared curing of the image-conforming coating on the printing form to achieve additional curing of this imaging layer and to increase impression constancy. During this thermal after treatment (fixing), the polymer of the imaging layer is heated above the glass temperature by the introduction of heat.
For this purpose, infrared irradiation of the layer produced on the printing form by the punctiform transferred thermal transfer material is performed, increasing adhesion to the surface of the printing form, as compared with non-irradiated regions on the printing form. Infrared irradiation of this type also introduces laser-induced heat. However, this infrared curing (since it is carried out over a large area) leads to uneven treating and pronounced heating of individual regions of the printing form. In particular, subject-dependent non-uniform heating of the imaging layer on the printing form occurs, i.e. the full-tone image regions heat up to a greater extent than graduated half-tone image regions. In an extreme case, this means that the property of improved impression constancy is distributed non-uniformly on the printing form. As a result, a loss of registry can occurs due to subject-dependent thermal expansion, or impression constancy cannot been achieved for the printing form.
The object of the present invention is to reversibly image a printing form by means of a thermal transfer material, to ensure a large impression having a uniform printing quality, without having to adapt the thickness of the layer of the thermal transfer material according to the desired impression. A further object of the present invention is to simplify the erasure of the image from the printing form.
Briefly stated, the present invention is a method for reversible imaging of a thermal transfer. First, imaging of the printing form is performed by punctiform activation. This involves image-controlled heating of the thermal transfer material on the transfer film, transfer of the image dots onto the surface of the printing form, followed by removal of the transfer film between the printing form and the image information transfer unit. In a further step, image-data-oriented activation is performed, (i.e. image-controlled heating of the surface of the printing form) so that the image information transferred in a pixel-like manner in the first step is heated, pixel by pixel, for a second time on the printing form. After the printing operation with the image information fixed in this way, the polymer parts can be removed from the printing form again.
Since the already transferred image information is activated a second time, preferably by a laser printing head without the transfer film being interposed, the imaging energy is increased and the image dots are cured more effectively, so that the impression constancy of the layer of thermal transfer material is clearly improved, without having to increase the thickness of the layer.
Preferably, a strip-like thermal transfer film consisting of a substrate layer, i.e. a carrier film or carrier strip, a substantially transparent heat-resistant plastic and a donor layer (i.e. the thermosensitive transferable layer) that is applied to the substrate layer are used as is known. The action of an energy source, preferably a laser beam from the rear side of the thermal transfer strip (i.e. from the uncoated side), induces heat in the donor layer and leads to softening and ultimately detachment of the thermal transfer material. Once transferred onto the printing form, the thermal transfer material immediately cools and adheres to the printing form due to the high heat capacity of the printing form material, for example metal. In this case, the thermal transfer film, in particular the layer of thermal transfer material, is preferably about 0.5 to 3 xcexcm thick.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.